Syntheses, crystal structures and thermoelectric properties of two new thallium tellurides: Tl4ZrTe4 and Tl4HfTe4

Abstract

Three new isostructural tellurides Tl₄MTe₄ with M = Zr and Hf were prepared from the constituent elements. Single crystal X-ray diffraction data analyses showed that these compounds belong to a new structure type, adopting the space group R with the unit cell dimensions of a = 14.6000(5) Å and c = 14.189(1) Å when M = Zr, and a = 14.594(1) Å and c = 14.142(3) Å when M = Hf (Z = 9). The structure consists of M atoms in distorted octahedra formed by Te atoms, which are face-condensed to oligomeric M₃Te₁₂ units. The structure refinement of the Zr compound revealed an additional site, occupied by 10% Zr, leading to a refined formula of Tl₄Zr_1.03Te₄. The electronic structure calculations predict semiconducting behavior for the stoichiometric compounds, which is in accordance with the experimental results. The thermoelectric properties of both compounds were determined, and the maximum values of the dimensionless figure-of-merit, ZT, were found to be 0.16 at 420 K for Tl₄ZrTe₄ and 0.09 at 540 K for Tl₄HfTe₄ in the measured temperature regime.